The following invention relates to a method and a structure for increasing the brightness of display panels using thin film electroluminescent (TFEL) devices.
TFEL panels are flat display screens which incorporate a thin film electroluminescent layer sandwiched by a pair of insulating layers and first and second electrode layers for producing a visible light output in the presence of an electric field generated by the electrode layers. Examples of such devices are shown in U.S. Pat. No. 4,719,385 "Multi-Colored Thin Film Electroluminescent Display," Jan. 12, 1988. The thin films that generate light in these devices are generally deposited on a substantially planar substrate such as glass, as shown in FIG. IA. The light emitting component is a phosphor film such as ZnS:Mn which may be formed by either evaporation or sputtering. For some phosphors such as ZnS:Tb sputtering is necessary in order to achieve the proper chemical composition of the thin film. A sputtered film, however, has a very smooth surface.
A phosphor film, typically, has a very high index of refraction which results in a small critical angle of reflection. If .theta..sub.c is the critical angle then sin .theta..sub.c equals n'/n where n' is the index of refraction of the medium outside the phosphor and n is the phosphor index of refraction. ZnS, for example, has a high index of refraction of 2.34. If n' is defined as the index of refraction of air (1.00) then the critical angle turns out to be 25.3.degree.. This means that most of the light impinging on the front of the device is lost due to internal reflection. This internal reflection results in light actually being piped out of the edges of the device which substantially reduces its luminance. Theoretically, for perfectly planar surfaces only 10% of the light actually emerges from the front.
Increasing the surface roughness of the thin films helps to alleviate this problem. However, it turns out that sputtered films have a flatter surface morphology than, for example, evaporated films. It is much more economical, however, to form the electroluminescent phosphor film by sputtering.
In the past, various approaches have been taken to increase the surface roughness of the films and thereby reduce the amount of light that is lost due to internal reflection. One such approach is shown in Levinson U.S. Pat. No. 4,774,435 in which the substrate is roughened by chemical etching, mechanical abrading or some other technique so that the topography of the substrate is replicated by the thin film layers which are later deposited. The same approach is used in Kane, et al., U.S. Pat. No. 4,728,581 where the electrodes first deposited on the substrate have a textured surface which propagates through the overlying layers. The problem with both of these approaches is that the devices become less reliable. Using the Kane approach, rough points on the transparent ITO conductor tend to pierce the insulating layers and cause destructive electrical breakdown. Also, high local electric fields are produced by the sharp crystalline facets of the roughened ITO which contribute to insulator breakdown. With the display disclosed in Levinson, the degree of surface roughness can become so high that tears and voids appear in the electrode layer ultimately causing breakdown. Also, modification of the glass substrate generates defects and causes trapped contamination on the surface of the glass, both of which later contribute to device failure.